Fix bug in snapshotting

[priyawadhwa]

When comparing layers in #204, the test for WORKDIR was failing because kaniko had an extra layer (It was creating an extra layer for the WORKDIR / command). I found two bugs when looking into it:

1. When snapshotting specific files, kaniko wasn't snapshotting the root directory, so when we ran WORKDIR / it appended a new layer even though nothing had changed.
2. when resolving env replacement kaniko was adding an extra backslash so / became //

This should fix both those bugs